Kant holds that in order to have knowledge of an object, a subject must be able to “prove” that the object is really possible—i.e., prove that there is neither logical inconsistency nor “real repugnance” between its properties. This is (usually) easy to do with respect to empirical objects, but (usually) impossible to do with respect to particular things-in-themselves. In the first section of the paper I argue that an important predecessor of Kant’s account of our ignorance of real possibility can be found in Locke’s Essay Concerning Human Understanding. In the middle sections I discuss the way in which our inability to prove the real possibility of things-in-themselves motivates Kant’s famous prohibition on certain kinds of knowledge-claims about them. In the final section I examine Hegel’s attempts to dissolve this problem of real repugnance and thereby remove an inherited obstacle to speculative knowledge of the supersensible.